Bicuculline and Picrotoxin Blockade of Positive Dorsal Root Potentials

Abstract

MOUNTING evidence suggests that GABA functions as a transmitter in the pathways mediating presynaptic inhibition in the spinal cord1–10 and cuneate nucleus11–14. In spite of recent attempts to locate a GABA synapse in these multi-synaptic pathways at the axo-axonic terminal4,13,15, the clarification of GABA's role in this system awaits the elucidation of the complex circuits mediating afferent terminal polarization. This complexity may include such features as tonic activity of interneurones and convergence of phasic inputs from different fibre groups. For example, Mendell16, recording spinal cord dorsal root potentials (DRPs), has observed that stimulation of high threshold afferent fibres from muscle nerve generates a positive going DRP, reflecting primary afferent hyperpolarization. From studies of the interactions between positive and negative DRPs he has postulated that the positive DRP results from a phasic inhibition of tonic activity in the pathway for the negative DRP17. This implies that the axoaxonic synapse is common to the pathways for both the positive and negative DRPs. Thus, if the only GABA synapse in this system is located at the axo-axonic junction, antagonists of GABA should block DRPs of both polarities.